In recent years, harbors have become much crowded with vessels of a large size, and therefore it has been required that tug boats should operate to move the vessel toward and away from the shore in a safe and rapid manner. For this reason, there have now been extensively used tug boats equipped with a Z-type propulsion apparatus which can easily vary the direction of propulsion over the range of 360 degrees. In such a Z-type propulsion apparatus, for example, shown in FIG. 1, the rotation of a main engine 1 is transmitted to an input shaft 6 via a universal joint 3, an intermediate shaft 4 and a universal joint 5 when a clutch 2 is connected, and the rotation of the input shaft 6 is transmitted to a propeller 12 attached to a propeller shaft 11 via bevel gears 7, 8, 9 and 10 so that the propeller 12 is rotated to advance the vessel. At the same time, the rotation of a hydraulic motor 14 controlled by a rotation control unit 13 is transmitted to a hollow rotary housing 17 via a worm gear 15 and a worm wheel 16 to angularly move the rotary housing 17 so that the direction of propulsion of the propeller 12 is changed to turn the vessel.
One conventional method of controlling the rotation of the Z-type propulsion apparatus is to control the speed of rotation of the hydraulic motor 14 with respect to the speed of rotation of the main engine 1. In such a conventional method, the main engine 1 drives a hydraulic pump so as to control the hydraulic motor 14. Alternatively, a motor drives the hydraulic pump to control the hydraulic motor 14. In the method in which the main engine 1 drives the hydraulic pump, the speed of rotation of the main engine 1 is detected and the angle of inclination of the hydraulic pump is controlled to be in inverse proportion to the speed of rotation of the main engine 1 so that the speed of rotation of the hydraulic motor 14 becomes constant. Alternatively, the speed of rotation of the hydraulic motor 14 is detected and the angle of inclination of the hydraulic pump is controlled so that the speed of rotation of the hydraulic motor 14 becomes constant.
Among these control methods, in the case where the vessel is equipped with the Z-type propulsion apparatus of the type by which a turning force exerted on the vessel is proportional to the square of the speed of travel of the vessel, it might be considered that the speed of rotation of the rotary housing is decreased to prevent an increase in the hydraulic drive force and an abrupt turning of the vessel when the speed of rotation of the main engine 1 is high. On the other hand, it might also be considered that when the speed of rotation of the main engine 1 is low, the speed of rotation of the rotary housing is increased to make use of the surplus of the hydraulic drive force and to rapidly turn the vessel.
However, vessels provided with such a Z-type propulsion apparatus are mostly used as tug boats. In the case where the tug boat has to carry out the mooring of a vessel, the speed of rotation of the main engine 1 is not always proportional to the speed of the boat or the number of revolution of the propeller. There are occasions when the speed of rotation of the main engine 1 is low while the speed of the boat is high and occasions when the speed of rotation of the main engine 1 is high while the speed of the boat is low. Therefore, in the method of controlling the speed of rotation of the rotary housing merely in inverse proportion to the speed of rotation of the main engine 1, the motor for driving the hydraulic pump is subjected to overload and is stopped by a safety device and also the hydraulic pump is damaged when the rotation of the main engine 1 is low while the speed of the boat is high (when the tug boat is tugged by the vessel). Also, when the speed of rotation of the main engine is high while the speed of the boat is low (when the tug boat tugs the vessel), a high speed of turning can not be achieved.
Another known rotation control system for controlling the turning of a vessel equipped with the Z-type propulsion apparatus is shown in FIG. 2. This system comprises a steering angle detector 22 for detecting the steering angle of a steering handle 21 for rotating a hollow rotary housing of the Z-type propulsion apparatus, a rotation angle detector 23 for detecting the rotation angle (follow-up angle) of the rotary housing 17, an error voltage generator 24 for comparing a detected value 81 of the steering angle detector 22 with a detected value 82 of the rotation angle detector 23 to output an error voltage, a rotation speed detector 25 for detecting the speed of rotation of the rotary housing 17, a servo circuit 25 for comparing the output of the error voltage generator 24 with the output of the rotation speed detector 25 to output a servo signal, a flow rate and direction control valve 27 such as an electromagnetic proportional control valve controlled by the servo signal, a hydraulic pump 29 connected to the flow rate and direction control valve 27 and driven by a motor 28, and a hydraulic motor 14 connected to the flow rate and direction control valve 27 and adapted to rotate the rotary housing 17 via a worm gear 15 and a worm wheel 16. As shown in FIG. 3, the error voltage generator 24 generates a voltage increasing in proportion to a difference of angle (.theta.1-.theta.2) when -A (A is a positive constant value) &lt;difference of angle (.theta.1-.theta.2)&lt;A is provided, and also it generates a negative or a positive constant voltage when the difference of angle (.theta.1-.theta.2).ltoreq.-A or the difference of angle (.theta.1-.theta.2).gtoreq.A is provided. Therefore, the servo circuit 26 controls the flow rate and direction control valve 27 so that the voltage generated by the error voltage generator 24 coincides with the output of the rotation speed detector 25. As a result, the hydraulic motor 14 is operated in accordance with the output of the error voltage generator 24 to rotate the rotary housing 17.
However, in the above-mentioned rotation control system, the speed of rotation of the rotary housing is controlled in accordance with the error voltage shown in FIG. 3 irrespective of the speed of the vessel, and is always constant except when the difference of angle (.theta.1-.theta.2) is in the vicinity of 0. Therefore, even if the steering handle 21 is angularly moved at the same angle, the turning of the vessel is varied depending on the speed of the vessel. More specifically, when the speed of the vessel is high, the vessel is turned at a high speed. This is dangerous. And, when the speed of the vessel is low, the turning speed is lowered so that a small turn can not be achieved, thereby affecting a steering performance of the vessel. Therefore, with the conventional system, the operator must change the manner of steering the steering handle 21 in accordance with the speed of the vessel. Thus, the steering operation is difficult and much skill is required.
It is therefore an object of this invention to provide a rotation control system for a Z-type propulsion apparatus which is capable of steering a vessel to achieve an optimum turn irrespective of the speed of the vessel.
Another object is to provide such a rotation control system which enhances the ability of high speed turning of the vessel.
A further object is to provide such a rotation control system which is capable of turning the vessel in a manner commensurate with the load of a propulsion device.
A still further object is to provide a rotation control system which is capable of controlling a plurality of Z-type propulsion apparatuses mounted on a vessel such as a multishaft vessel and is capable of steering the vessel to achieve an optimum turn.